All coins make the same amount of profit per day. People use monitors that monitor profitability and they switch to the most profitable coin to make an extra %'s.
No secret to this. Unless the coin is unlisted and then nobody mines it, but that's a gamble.
They don't all really earn the same amount. If you choose the right one and the value goes up dramatically then it's more profitable. I remember getting in on Zetacoin years ago and making a tidy profit on that, and that was back in the old Block Erupter days